West Nile virus (WNV) is a highly virulent human pathogen of the central nervous system (CNS) and the most common cause of epidemic encephalitis in the United States. There are no vaccines or specific antiviral treatments available for WNV, and accomplishing this requires a more complete understanding of its pathogenesis. In the CNS, myeloid cells, including infiltrating inflammatory monocytes, macrophages, and activated microglia, comprise the majority of the cellular response to WNV encephalitis. Currently, the specific role of each myeloid subset in regulating viral pathogenesis is largely unknown. The primary goal of this proposal is to understand the fundamental biology of myeloid cell populations during WNV infection. To do this, we will 1) establish the mechanisms and significance of WNV-induced monocyte mobilization, 2) define the migration and function of CCR2-expressing monocytes in the WNV-infected CNS, and 3) characterize the neuropathogenic role of CX3CR1-expressing microglia during WNV encephalitis. Detailed knowledge of the specific roles of myeloid cell subsets will provide insights into their fundamental biology and their contribution to host defense. Furthermore, our results will provide the basis for novel intervention strategies targeting myeloid cell subsets in the CNS, when the need for effective treatments is most critical.